The main olfactory and vomeronasal (VN) systems are the major nasal chemosensory systems of most terrestrial vertebrates. A major question that has eluded solution for both systems is the mechanism of stimulus coding. One of the prominent current hypotheses proposes that sensory receptor cells with different complements of receptor proteins on their distal processes send their axons to segregated portions of the olfactory bulbs. In the past several years our laboratory has demonstrated that the accessory olfactory bulb (AOB) of the grey, short-tailed opossum, Monodelphis domestica is heterogeneous. A number of "markers" are expressed in or strain more strongly the rostral AOB than the caudal AOB. These "markers" include olfactory marker protein (OMP), the lactin Vicia villosa (VVA), NADPH-diphorase, and Gi2a. In addition, we have observed that in the AOB Goa is located predominantly in its posterior subdivision. This heterogeneity in the chemoarchitecture of the AOB may reflect a fundamental organizational dichotomy within the vomeronasal system that corresponds to a functional dichotomy and it is the goal of the studies described herein to pursue this problem. This goal will be accomplished by 1. determining whether the vomeronasal epithelium contains separate population of receptor cells t hat are distinguishable with the "markers" that differentially stain the A OB; 2. determining whether the receptor cells that project to the rostral AOB; 3. determining whether two populations of receptor cells, already discriminated with antibodies to the two G proteins, are of different ages; 4. determining whether athe mitral/tufted cells of one subdivision of the AOB have principal dendrite restricted to that subdivision; and 5. determining whether athe centripetal projections of the anterior AOB differ from those of the posterior AOB.